1. Field of the Invention
The present invention relates to a processing apparatus for converting a television scanning signal, and more particularly, to an apparatus employing motion-adaptive processing to create a progressive scanning signal with improved image quality from an interlaced television scanning signal.
2. Description of the Related Art
According to a known motion-adaptive technique for converting a scanning line number of a television signal, are formed two types of interpolation signals, a motion image-matching interpolation signal and a still image-matching interpolation signal. These signals are obtained through intra-field interpolation processing and inter-field interpolation processing, respectively. A circuit employing in this technique a scanning line interpolation signal is selected in accordance with the movement of the image. This conversion technique is illustrated in, for example, Japanese Patent Application Kokai Publication No. 4-3151 and will be explained below to reference with FIG. 1. In this illustrated circuit, an input interlaced scanning signal is converted to a progressive scanning signal to having twice the number of scanning lines as the input signal by combining an interpolation signal formed from the input signal and the input signal per se. Having two interpolation signal forming routes are provided, one in which a motion image-matching interpolation signal is formed and in which a still image-matching interpolation signal is formed. The motion image-matching interpolation signal is formed from canning line signals within the same field and the still image-matching interpolation signal is formed from scanning signals from a preceding field and/or a following field. A motion detecting signal generated by a motion detection circuit causes either the still image-matching interpolation signal or the motion image-matching interpolation signal to be properly selected in accordance with a still image mode or motion image mode of the input signal.
In FIG. 1, an interlaced scanning signal is supplied to an input terminal 312. The input signal is supplied to a field delay circuit 314 and to a subtracter 319. The output of the field delay circuit 314 is input to a field delay circuit 315, a line delay circuit 316 giving an amount of delay corresponding to one horizontal period, adder 317 and time compression circuit 324. The signal to the time compression circuit 324 is of a direct type.
The output of the field delay circuit 315 is input to the subtracter 319 and coefficient unit 322 and the output of the coefficient unit 322 is input to an adder 323. The output of the line delay circuit 316 is input to the adder 317. The output of the adder 317 is sent through 1/2 coefficient unit 318 and then through a coefficient unit 321 to the adder 323. The output of the adder 323 is of an interpolation type and is input to a time compression circuit 325 where it is time-compressed. The output of the time compression circuit 325 is supplied to one contact of a switch 326. The output of the time compression circuit 324 is supplied to the other contact of the switch 326. The selected output of the switch 326 is derived from an output terminal 313.
The output of the subtracter 319 is input to a motion detector 320. The motion detection signal of the motion detector 320 determines whether the input signal corresponds to the motion image mode or the still image mode and delivers a corresponding signal to the control terminals of the coefficient units 322 and 321.
A route consisting of line delay circuit 316, adder 317, 1/2 coefficient unit 318 and coefficient unit 321 is provided for generating the motion image-matched interpolation signal. An alternative route consisting of field delay circuit 315 and coefficient unit 322 is provided for generating the still image-matching interpolation signal. If the movement of the image is greater, the coefficients of coefficient units 321 and 322 are made greater and smaller, respectively, so that the adder 323 produces the motion image-matching interpolation signal. If, on the other hand, the still image is involved, the coefficients of the efficient units 321 and 322 are made smaller and greater, respectively, so that the adder 323 produces the still image-matching interpolation signal. Both the direct type signal and the interpolation type signal are time-compressed and alternately and selectively supplied through the switch 326 to the output terminal. By doing so, a progressive scanning signal emerges from the output terminal 313 as an output signal having twice the number of scanning lines as the input interlaced scanning signal.
In the above conversion circuit, when conversion is performed by the inter-field interpolation processing for the still image mode, it is advantageous to entirely avoid a vertical-direction lowered response and eliminate an interline flicker on the still image. Furthermore when the motion detector erroneously determines a still image mode; in spite of the input signal being in the motion image creates mode, the output signal produced by the inter-field interpolation processing a degeneration of the image. In other words, when the motion detector incorrectly determines that the input signal is in a still image mode, then an interpolation signal will be created from the preceding field. Thus, in this case, if inter-field image movement occurs, a residual image will emerge on the screen, thus leading to a degenerated image. This defect is visually very perceptible.
In the motion image mode, intra-field interpolation processing is carried out, but only 240 scanning lines information are used for the interlaced scanning signal. For this reason, the vertical image resolution is limited to 120 cph (cycle per picture height) according to the Sampling Theorem Principle. For the filtering characteristic of a real circuit, the effective resolution is even further reduced.
According to the example shown in FIG. 1, the input signal may be considered as being 2-line interval sampling data (240 TVL/PH =240 TV Line Per Picture Height). Thus, for the input signal, if scanning line interpolation is carried out using "0" data, the sampling data is seemingly up-converted to one-line interval sample data (480 TVL/PH), this being equivalent to the processing made by a vertical lowpass filter equipped with three taps (coefficients: 0.5, 1, 0, 0.5). The filter characteristic of the intra-field interpolation processing corresponds to a cosine-squared characteristic with vertical space frequencies 120 cph and 240 cph at -6db and a null point, respectively. The above filter characteristic means that, first, those components near the 120 cph (240 TVL/PH) are attenuated due to the pass band of the filter not being flat, Therefore, the frequency characteristic of the processing exerts a greater influence on the vertical sharpness, that is, the image quality is prominently degraded and highly blurred. Second, the above filter characteristic means that, high-frequency components of over 120 cph are not attenuated by the filter and emerge as aliasing components, further degenerating image quality.
Let it be assumed that, when a still image having adequate vertical high-frequency components is entered, motion image-mode processing (intra-field interpolation processing) is erroneously carried out due to an operational error of the motion detector. In this case, image quality becomes markedly degenerated with the emergence of greater aliasing components on the screen.
The selection of inter-field interpolation or intra-field interpolation processing is controlled by the determination of the motion detector. Since the input signal is composed of an interlaced scanning signal, offset sampling is made between the fields. Usually, the motion components are detected by means of an inter-frame difference calculation. The motion detector is comprised of a band pass filter having a cosine characteristic with a peak at 15 Hz and a level "0" at 0 Hz and 30 Hz in a temporal direction frequency region. Originally, the motion detection must allow for detection of a temporal components ranging from over 0 Hz up to 30 Hz. However, the 30 Hz motion component essentially cannot be detected because the input signal is used for a interlaced scanning and, in other words, there may be some cases where the motion detection cannot account for a quick motion image.
With the conventional scanning line number converting apparatus, when, as set out above, an operation error arises in the motion detection for motion-adaptive processing, the resulting image quality is greatly degenerated. Furthermore, a great difference in image quality arises between the still image mode and the motion image mode. That is, even in the case where no operation error occurs upon the detection of the motion, image quality is not adequate in the motion image mode. It is, therefore, theoretically impossible to avoid detection and operation errors with the conventional motion circuit. In view of this situation, more efforts are demanded to further improve image quality that is, the scanning line number conversion output obtained through the motion-adaptive processing.